diff --git a/lib/libsys/clock_gettime.2 b/lib/libsys/clock_gettime.2
--- a/lib/libsys/clock_gettime.2
+++ b/lib/libsys/clock_gettime.2
@@ -27,7 +27,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd June 28, 2024
+.Dd August 10, 2024
 .Dt CLOCK_GETTIME 2
 .Os
 .Sh NAME
@@ -99,6 +99,15 @@
 Returns the execution time of the calling process.
 .It Dv CLOCK_THREAD_CPUTIME_ID
 Returns the execution time of the calling thread.
+.It Dv CLOCK_TAI
+Increments in SI seconds like a wall clock.
+It uses a 1970 epoch and implements the TAI timescale.
+Unlike CLOCK_REALTIME, does not include leap seconds.
+Behavior during a leap second is not defined and depends on the underlying
+system clock implementation.
+Will return EINVAL if the current offset between TAI and UTC is not known,
+which may be the case early in boot before NTP or other time daemon has not
+yet synchronized.
 .El
 .Pp
 The clock IDs
diff --git a/sys/kern/kern_ntptime.c b/sys/kern/kern_ntptime.c
--- a/sys/kern/kern_ntptime.c
+++ b/sys/kern/kern_ntptime.c
@@ -153,7 +153,8 @@
 
 static int time_state = TIME_OK;	/* clock state */
 int time_status = STA_UNSYNC;	/* clock status bits */
-static long time_tai;			/* TAI offset (s) */
+long time_tai;			/* TAI offset (s) */
+bool time_tai_set = false;		/* Whether time_tai has been set */
 static long time_monitor;		/* last time offset scaled (ns) */
 static long time_constant;		/* poll interval (shift) (s) */
 static long time_precision = 1;		/* clock precision (ns) */
@@ -388,8 +389,10 @@
 			time_constant = ntv->constant;
 	}
 	if (modes & MOD_TAI) {
-		if (ntv->constant > 0) /* XXX zero & negative numbers ? */
+		if (ntv->constant > 0) { /* XXX zero & negative numbers ? */
 			time_tai = ntv->constant;
+			time_tai_set = true;
+		}
 	}
 #ifdef PPS_SYNC
 	if (modes & MOD_PPSMAX) {
diff --git a/sys/kern/kern_tc.c b/sys/kern/kern_tc.c
--- a/sys/kern/kern_tc.c
+++ b/sys/kern/kern_tc.c
@@ -1038,6 +1038,8 @@
 /* Internal NTP status and error estimates. */
 extern int time_status;
 extern long time_esterror;
+extern long time_tai;
+extern bool time_tai_set;
 
 /*
  * Take a snapshot of sysclock data which can be used to compare system clocks
@@ -1139,6 +1141,11 @@
 			getboottimebin(&boottimebin);
 			bintime_add(bt, &boottimebin);
 		}
+		if (!(flags & FBCLOCK_LEAPSEC)) {
+			if (time_tai == 0)
+				return (EINVAL);
+			bt->sec += time_tai;
+		}
 		break;
 #ifdef FFCLOCK
 	case SYSCLOCK_FFWD:
diff --git a/sys/kern/kern_time.c b/sys/kern/kern_time.c
--- a/sys/kern/kern_time.c
+++ b/sys/kern/kern_time.c
@@ -49,6 +49,7 @@
 #include <sys/proc.h>
 #include <sys/posix4.h>
 #include <sys/time.h>
+#include <sys/timeffc.h>
 #include <sys/timers.h>
 #include <sys/timetc.h>
 #include <sys/vnode.h>
@@ -318,7 +319,10 @@
 kern_clock_gettime(struct thread *td, clockid_t clock_id, struct timespec *ats)
 {
 	struct timeval sys, user;
+	struct sysclock_snap clk;
+	struct bintime bt;
 	struct proc *p;
+	int err;
 
 	p = td->td_proc;
 	switch (clock_id) {
@@ -329,6 +333,14 @@
 	case CLOCK_REALTIME_FAST:
 		getnanotime(ats);
 		break;
+	case CLOCK_TAI:
+		sysclock_getsnapshot(&clk, 0);
+		err = sysclock_snap2bintime(&clk, &bt, clk.sysclock_active,
+		    (clk.sysclock_active == SYSCLOCK_FFWD) ? FFCLOCK_LERP : 0);
+		if (err)
+			return (err);
+		bintime2timespec(&bt, ats);
+		break;
 	case CLOCK_VIRTUAL:
 		PROC_LOCK(p);
 		PROC_STATLOCK(p);
diff --git a/sys/sys/_clock_id.h b/sys/sys/_clock_id.h
--- a/sys/sys/_clock_id.h
+++ b/sys/sys/_clock_id.h
@@ -74,6 +74,10 @@
 #define	CLOCK_PROCESS_CPUTIME_ID 15
 #endif /* __POSIX_VISIBLE >= 199309 */
 
+#ifdef __BSD_VISIBLE
+#define	CLOCK_TAI		16
+#endif
+
 /*
  * Linux compatible names.
  */
diff --git a/sys/sys/timeffc.h b/sys/sys/timeffc.h
--- a/sys/sys/timeffc.h
+++ b/sys/sys/timeffc.h
@@ -89,7 +89,7 @@
  *			of the kernel tick timer (1/hz [s]).
  * FFCLOCK_LERP:	Linear interpolation of ffclock time to guarantee
  *			monotonic time.
- * FFCLOCK_LEAPSEC:	Include leap seconds.
+ * {FB|FF}CLOCK_LEAPSEC: Include leap seconds.
  * {FB|FF}CLOCK_UPTIME:	Time stamp should be relative to system boot, not epoch.
  */
 #define	FFCLOCK_FAST		0x00000001
@@ -100,6 +100,7 @@
 
 #define	FBCLOCK_FAST		0x00010000 /* Currently unused. */
 #define	FBCLOCK_UPTIME		0x00020000
+#define	FBCLOCK_LEAPSEC		0x00040000
 #define	FBCLOCK_MASK		0xffff0000
 
 /*